That's a really good point, and reminds me of some thoughts I had last time I was at the white board explaining the difference between composition and inheritance. I was irritated as crap to be doing it, but I also realized that my irritation should be with myself. Even if I'm not a rock-star programmer with huge contributions to open source, I should, by this point, have a body of work that speaks louder than a technical interview.
Part of the problem is that many programmers still don't get to do open source work (I couldn't have revealed the source code to what I was working on at the time in an interview), whereas many other fields (design in particular) provide more opportunity to show off your work publicly. But there is a choice here, and as you've pointed out, it's important to make sure that at least some of your work is publicly available.
Even "demo" sites don't strictly need to be demo anymore - a small "real" app can be created, deployed, and used, even if the main purpose was to show off skills.
Part of the problem is that many programmers still don't get to do open source work (I couldn't have revealed the source code to what I was working on at the time in an interview), whereas many other fields (design in particular) provide more opportunity to show off your work publicly. But there is a choice here, and as you've pointed out, it's important to make sure that at least some of your work is publicly available.
Even "demo" sites don't strictly need to be demo anymore - a small "real" app can be created, deployed, and used, even if the main purpose was to show off skills.